Effects of variable thermophysical properties on the layered structure of incompressible turbulent channel flows

Abstract

Abstract

This study employs a scaling-patch framework to investigate how variable thermophysical properties modify the layered structure and scaling behaviour of turbulent channel flows. Direct numerical simulation data of wall-heated and wall-cooled turbulent gas flows are analysed to quantify the effects of wall-normal variations in density, viscosity and thermal conductivity on momentum and heat transport across distinct flow regions. The results suggest the presence of a near-wall variable-property sublayer, in which viscous effects associated with wall-normal viscosity gradients contribute appreciably to the local momentum balance. A dimensionless parameter is introduced to characterise the influence of the wall-normal viscosity gradient at the wall, and a Stratford–Mellor-type scaling is proposed for the corresponding force balance within this sublayer. In the adjacent inner layer, where viscous forces and Reynolds shear stress gradients dominate, both terms are noticeably affected by property variations, highlighting the limitations of conventional inner scaling for variable-property flows. In the outer region, while the flow retains structural similarities to constant-property turbulence, traditional outer scaling does not fully collapse mean velocity and temperature deficit profiles, motivating a new outer scaling that improves data collapse across different thermal wall conditions. Finally, the scaling-patch framework is used to interpret these results. Within this framework, the Trettel–Larsson transformation emerges naturally from the local force balance when semi-local scaling is applied to the Reynolds shear stress, providing a consistent perspective on how variable-property effects influence wall-bounded turbulence.